This invention relates to a method of determining the location of first and second devices.
Recent progress in the field of GPS has enabled GPS receivers to be produced relatively cheaply leading to their widespread adoption and use. For example, one might envisage a home networking environment in which is provided a mobile telephone with GPS capability for providing its location to a cellular telephone network operator in the event of an emergency call; a TV with GPS capability for providing TV access control, say as described in U.S. Pat. No. 5,621,793; and a personal computer with GPS and Internet capability for retrieving location specific information from a web site, say a local weather report.
In such a home networking environment, three GPS receivers are provided, each returning substantially the same location. However, especially indoors, it is unlikely that all three GPS receivers will each be able to acquire the four GPS space vehicle (SV) signals normally required to obtain a position fix, or at least not all of the time. One reason for this is obscuration of the GPS signals by buildings, walls and other urban paraphernalia.
It is therefore an object of the present invention to provide a method of determining the location of a device which is more effective indoors.
According to a first aspect of the present invention, there is provided a method of providing an estimate of the location of first and second devices which are located near to each other, the method comprising the steps of:
obtaining at least one range measurement from the first device to a corresponding reference point;
obtaining at least one range measurement from the second device to a corresponding reference point: and
calculating an estimate of the location of the devices using range measurements obtained with respect to both the first and second devices,
and wherein at least one range measurement obtained from the second device was obtained with respect to a reference point to which no range measurements were obtained from the first device.
Such a method enables an estimate of the location of the devices to be calculated in circumstances where it is impossible to unambiguously determine the location of the first device, or the location of the second device using only range measurements obtained from the second device.
In particular, but not exclusively, the or each range measurement obtained with respect to the second device and for each such range measurement an indication of the associated reference point, e.g. in the form of a position co-ordinate, are provided to the first device for calculating an estimate of the location of the devices, preferably using a wireless communications link.
Where the range measurement information is provided from the second device to the first device using a short range communications link such as Bluetooth, one can safely assume that the devices are close together and that the resultant position fix is a good estimate of the true position of the first second devices. Also, having calculated an estimate of the location of the devices at the first device, the estimate may then be provided to the second device.
Preferably, the first and second devices have reciprocal functionality in that the or each range measurement obtained with respect to the first device and for each such range measurement an indication of the associated reference point may also be provided to the second device for calculating an estimate of the location of the devices.
Alternatively, the each range measurement obtained with respect to both the first an d second devices and for each such a range measurement an indication of the associated reference point may be provided to a third device for calculating an estimate of the location of the first and second devices.
At least one range measurement obtained in respect of either the first or the second device may be a pseudorange range measurement.
For example, the position determining means of the second device may include a GPS receiver wherein at least two range measurements obtained with respect to the second device are pseudorange measurements from a GPS SV. Also, the position determining means of the first device may also include a GPS receiver wherein at least two range measurements obtained with respect to the first device are pseudorange measurements from a GPS SV; and wherein at least 5 pseudorange measurements are obtained with respect to either the first or the second device in total. I.e. sufficient to obtain a position fix.
NB. In order to calculate an estimate of the location of the first device using pseudorange measurements obtained with respect to both first and second devices, clock errors of both the first and second devices must be resolved, i.e. the clock errors between the synchronized GPS SVs and each device. Fortunately, however, pseudoranges may be more conveniently obtained than absolute range measurements and this may justify the extra computation.
Alternatively, where the position determining means of the first and second device both includes a GPS receiver and the first and second device are synchronized, a single pseudorange measurement from a GPS SV obtained with respect to either the first or second device may contribute to the position determination. Without synchronicity, the benefit of a device obtaining a single pseudorange is offset by the extra clock error introduced.
At least one range measurement obtained with respect to the first and/or the second device may be a range measurement to a ground based reference point. This information may then be combined with either other ground and or space based range or pseudorange measurements in order to obtain a position fix.
For example, this may be an absolute range measurement determined by a time-of-arrival measurement with respect to a fixed base station, and may be conveniently provided when the first and/or second device is a mobile cellular telephone able to communicate using a wireless communications link which is independent of any cellular basestation network. Synchronicity between a mobile cellular telephone and the cellular telephone network basestation with which it is registered is relatively easy to achieve, and thus an absolute range measurement may be readily obtained.
Also provided in accordance with the first aspect of the present invention is a device able to provide an estimate of its location comprising ranging means for obtaining at least one range measurement from the device to a first reference point and location information of the first reference point; a receiver for receiving ranging information relating to at least one range measurement from another device, located near the device and thus essentially at the same location, to a second reference point and location information of the second reference point; and a processor for calculating an estimate of the location of the device using both of the range measurements obtained by its ranging means and from the ranging information, and wherein at least one range measurement obtained from the ranging information was obtained with respect to a reference point to which no range measurements were obtained by the ranging means.
According to a second aspect of the present invention, there is provided a method of determining the position of a first GPS receiver with the assistance of a second, portable GPS receiver comprising the steps of:
acquiring GPS signals in the first GPS receiver and deriving GPS signal information therefrom;
providing the GPS signal information from the second GPS receiver to the first GPS receiver; and
acquiring GPS signals in the first GPS receiver using the GPS signal information provided by the second GPS receiver, and determining the position of the first GPS receiver therefrom.
The term xe2x80x9cportablexe2x80x9d is intended to mean that it could be conveniently carried from place to place by a human and would normally be carried from place to place by a human. As such, portable would include a mobile telephone, a personal digital assistant (PDA), a palm-top computer or lap-top computer, all incorporating a GPS receiver.
Preferably, the first and second GPS receivers have reciprocal functionality in that the first GPS receiver may also acquire GPS signals and derive GPS signal information therefrom, and provide GPS signal information to the second GPS receiver; and that the second GPS receiver may acquire GPS signals using the GPS signal information provided by the first GPS receiver.
Also, it is desirable that the first GPS receiver provide this assistant whilst necessarily being able to determine its own position, i.e. not be able to acquire the four GPS signals normally necessary to do so.
Also provided in accordance with the second aspect of the present invention is a GPS receiver unit 11.